This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Natural gas reservoirs may often contain high levels of acid gases, such as carbon dioxide (CO2). In these cases, a cryogenic process may provide an efficacious way to separate the acid gases from the methane. The cryogenic process could include a simple bulk fractionation, a Ryan-Holmes process, or a more complex cryogenic fractionation process. The cryogenic processes separate methane from CO2 by condensation and fractionation, and can produce the acid gas in a liquid phase for efficient disposal via pumping. However, in the cryogenic processes, hydrocarbons heavier than methane, e.g., natural gas liquids (NGLs), are separated with the CO2 in a single liquid stream. Often, the CO2 will be immediately reinjected for disposal.
In some locations, a natural gas reservoir contains high levels of CO2. In such locations, it may be desirable to use a cryogenic process to separate the CO2 from the methane. The cryogenic process could be a simple bulk fractionation process, a Ryan-Holmes process, or a Controlled Freeze Zone (CFZ™) process. These processes separate methane from CO2 by condensation or fractionation, and can provide the CO2 as a liquid for efficient disposal. However, in these processes, the NGLs are also condensed and separated with the CO2. Normally, the CO2 will be reinjected for disposal. However, the NGLs are valuable. Thus, it may be desirable to recover the NGLs for sale.
Separation of the NGLs can be performed by fractionation. However, ethane forms an azeotropic mixture with CO2, as discussed with respect to FIG. 1. Such an azeotropic mixture may prevent separation by normal techniques.
FIG. 1 is a temperature-composition phase plot 100 showing the equilibrium concentrations of CO2 in a mixture with ethane at 4,137 kilopascals (kPa, 600 psia). The x-axis 102 indicates the mole fraction of CO2, while the y-axis 104 represents the temperature in degrees Celsius (° C.). The concentration of the CO2 in the vapor phase 106 matches the concentration of the CO2 in the liquid phase 108 at about 70% CO2/30% ethane, as indicated by an arrow 110. This prevents separation-by-fractionation across the azeotrope (left to right, or right to left).
FIG. 2 is a temperature-composition phase plot 200 showing the equilibrium concentrations of CO2 in a mixture with ethane at 689.5 kPa (100 psia). Like numbered items are as described with respect to FIG. 1. As this plot 200 shows, concentration of the CO2 in the vapor phase 106 approaches the concentration of the CO2 in the liquid phase 108 at about 60% CO2/40% ethane, as indicated by an arrow 202. This prevents separation-by-fractionation across the azeotrope (left to right, or right to left). As these plots 100 and 200 indicate, complete separation by fractionation cannot be achieved without some additional separation processes.
Current practices for CO2 I ethane separation includes various methods. For example, a heavy component (lean oil) can be added, which preferentially absorbs the ethane. This is called “extractive distillation.” As another example, two-pressure fractionation can be used to exploit the small difference in the azeotropic composition between different pressures, for example, using two fractionators to fractionate at both 4,137 kPa and 689.5 kPa. However, this technique utilizes a very large recycle stream and large fractionation systems. Further, the compressors needed to move from the low pressure to the high pressure column make the technique very energy intensive. Methods to exploit other physical and chemical properties can be used in conjunction with fractionation to achieve separation. These methods may include the use of amines in a chemical reaction with CO2, gas permeation membranes, or molecular sieves.
For example, U.S. Pat. No. 4,246,015, to Styring, discloses a method for separating CO2 and ethane based on washing ethane from frozen CO2. The separation is accomplished by freezing the CO2 in a CO2 and ethane mixture and washing the ethane from the solid CO2 with a liquid hydrocarbon, e.g., lean oil, having at least three carbon atoms. The freezing process may be preceded by distillation of a CO2-ethane mixture to form an azeotropic mixture. A subsequent distillation may be used to separate the wash hydrocarbon from the CO2. In addition, if desired, the ethane-wash hydrocarbon mixture may be similarly separated in a subsequent distillation stage. However, the use of lean oil results in the contamination of the ethane, and utilizes large amounts of heat for regenerating the lean oil. Further, high lean oil circulation rates are needed, and the ethane is not able to be completely recovered.
U.S. Patent Application Publication No. 2002/0189443, by McGuire, discloses a method of removing CO2 or hydrogen sulfide (H2S) from a high pressure mixture with methane. The high pressure mixture is expanded through a flow channel having a convergent section followed by a divergent section with an intervening throat that functions as an aerodynamic expander. The flow channel is operated at temperatures low enough to result in the formation of solid CO2 and solid H2S particles, which increases the efficiency of CO2 and H2S removal. However, such techniques rely on the use of a high pressure mixture with a high proportion of methane and a relatively low proportion of CO2. In some cases, it may be desirable to remove CO2 from a mixture that contains a large proportion of CO2, e.g., more than around 40% CO2.
International Patent Publication No. WO/2008/095258, by Hart, discloses a method for decreasing the concentration of CO2 in a natural gas feed stream containing ethane and C3+ hydrocarbons. The process involves cooling the natural gas feed stream under a first set of conditions to produce a liquid stream including CO2, ethane, and C3+ hydrocarbons and a gas stream having a reduced CO2 concentration. The liquid stream is separated from the gas stream, and C3+ hydrocarbons may be separated from the liquid stream. The gas stream is then cooled under a second set of conditions to produce a sweetened natural gas stream and a second liquid containing liquid CO2 and/or CO2 solids. The sweetened natural gas stream is separated from the second liquid. However, this technique relies on the use of amines, membranes, and molecular sieves, which release the CO2 as a vapor at low pressure and increase the cost of disposal.
International Patent Publication No. WO/2009/084945, by Prast, discloses a method and assembly for removing and solidifying CO2 from a fluid stream. The assembly has a cyclonic fluid separator with a tubular throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device. The separation vessel has a tubular section positioned on and in connection with a collecting tank. A fluid stream with CO2 is injected into the separation assembly. A swirling motion is imparted to the fluid stream so as to induce outward movement. The swirling fluid stream is then expanded such that components of CO2 in a meta-stable state within the fluid stream are formed. Subsequently, the outward fluid stream with the components of CO2 is extracted from the cyclonic fluid separator and provided as a mixture to the separation vessel. The mixture is then guided through the tubular section towards the collecting tank, while providing processing conditions such that solid CO2 is formed. Finally, solidified CO2 is extracted. However, this technique may not provide for an acceptable degree of separation of the CO2, since the CO2 may form an azeotrope with the other components of the fluid stream as the fluid stream flows through the tubular section towards the collecting tank.